In recent years, since the communication areas of a plurality of networks having different communication schemes are overlapped, mobile stations are often connected to any of these networks and provided with communication services. Examples of the above-mentioned communication schemes include a GSM (Global System for Mobile Communication), a UTRAN (Universal Terrestrial Access Network), and an E-UTRAN (Evolved UTRAN).
With the recent increase in traffic, an attempt has been made to reduce the load of a radio access network equipped with macro base stations by locating small mobile phone base stations or wireless LAN (WLAN: Wireless Local Area Network) base stations. Another attempt has been made to reduce the load of a core network by sending the traffic of mobile stations from small wireless base stations or wireless LANs to the external network (Internet) without a core network which is host to a radio access network. A network established by combining networks having different communication schemes, as described above, is called a HetNet (Heterogeneous Network).
Regarding the HetNet, a method for cooperation between a wireless LAN and a mobile phone network is described in NPL 1 (3GPP (3rd Generation Partnership Project) TR 37.834 v12.0.0). In this method, a mobile station measures the radio qualities of signals transmitted from both the wireless LAN and the mobile phone network. The mobile station then determines a network to be connected, on the basis of factors such as the measurement results of the radio qualities in accordance with a policy or a threshold set from the mobile phone network or an instruction from the mobile phone network.
PTL 1 (Japanese Unexamined Patent Application Publication No. 2004-260444) discloses a method for implementing a seamless handover while reducing the processing load in a HetNet. In this method, a mobile station measures the radio quality of a signal from a radio access station in each of a plurality of networks having different communication schemes and determines a radio access station to be connected on the basis of the measurement results. A network controller which is host to a radio access station controls the handover operation of a mobile station on the basis of a radio access station to be connected, a network accommodating the radio access station, and the moving speed of the mobile station.
In the above-mentioned methods described in NPL 1 and PTL 1, the mobile station needs to measure the radio quality of a signal from a network having each communication scheme. Therefore, the number of signals, the radio qualities of which are to be measured by the mobile station, increases, and the power consumption of the mobile station, in turn, increases.
To overcome this issue, PTL 2 (International Publication No. WO 2007/080627) discloses a method for suppressing the increase of the power consumption of a mobile station in a wireless communication system in which the communication areas of a 3rd generation mobile communication system (3G), a 4th generation mobile communication system (4G), and Super 3G (S3G) intermediate between 3G and 4G are overlapped. In this method, the mobile station selects signals, the radio qualities of which are to be measured, from signals of each of 3G, S3G, and 4G in accordance with the remaining level of the battery and determines a connection destination from networks corresponding to the signals, the radio qualities of which have been measured. Selecting signals, the radio qualities of which are to be measured, can suppress the increase of the power consumption of the mobile station compared to the case where the radio qualities of signals of each of 3G, S3G, and 4G are measured.
The problem that the power consumption of the mobile station increases due to an increase in number of objects, the radio qualities of which are to be measured, is not limited to a HetNet. This problem may also arise in CA (Carrier Aggregation) defined in NPL 2 (3GPP TS 36.300 v12.1.0).
CA is a technique for transmitting and receiving data using a maximum of five component carriers (to be referred to as CCs hereinafter) having a maximum of 20 MHz. In CA, the mobile station needs to measure the radio qualities of signals having a plurality of frequency bands to use CCs having different frequencies in combination, thus the power consumption of the mobile station increases.
The above-mentioned problem sometimes arise even in DC (Dual Connectivity) defined by NPL 3 (3GPP TR 36.842 v12.0.0).
In DC, a Master eNB (evolved NodeB) (to be referred to as an MeNB hereinafter) and a Secondary eNB (to be referred to as an SeNB hereinafter) are defined. The mobile station exchanges control data with the far MeNB and exchanges user data with the near SeNB. In DC, the mobile station needs to measure the radio qualities of signals transmitted in cells respectively formed by the MeNB and the SeNB, thus the power consumption of the mobile station increases.